In a wireless communication system including wireless terminals (MSs) and wireless base stations (BSs) that provide wireless communication zones, each MS can communicate with another MS via one of those BSs if the MS is in a range (namely, service area) where the MS can communicate wirelessly with that BS.
In such a wireless communication system, the communication breaks down generally when an MS moves out of the area where the radio wave of the communicating BS can reach (namely, out of the service area of that BS). Therefore, the wireless communication system generally has handover (HO) function, with which the communicating BS is switched to another as the MS moves.
During the above-mentioned handover process, the link-layer communication between the MS and the BS momentarily breaks down, and therefore, the MS can not transmit packets to the communication-party MS temporarily. Such packets that can not be transmitted to the communication party are generally dropped in the course of their transmission.
For example when an MS communicates with a communication party according to TCP (Transmission Control Protocol), the MS measures time (namely, RTT: Round Trip Time) between the transmission of a packet and the reception of the ACK (ACKnowledgement) with respect to that packet and calculates the time-out value (RTO: Retransmission TimeOut) of the data-retransmission timer at the time of the retransmission control on the basis of that RTT. When the ACK is not received in a communicating process even after the timer exceeds the present RTO value (namely, time-out), a congestion control is (for example, a control to lower the communication speed considerably at which the MS communicates with a communication party) exercised between the MS and a communication party. Therefore, in such a case, the throughput may probably be lowered because of slow-start algorithm after completion of the handover process.
In this regard, Non-Patent Documents 1 (IEEE Std 802.16e.-2005 and IEEE Std 802.16.-2004/Cor 1-2005 (Amendment and Corrigendum to IEEE Std 802.16-2004)) and 2 (IEEE Std 802.16.-2004 (Revision of IEEE Std 802.16.-2001)) listed below disclose, for example, a method to inhibit drop of the packets during a handover process by buffering the packets transmitted from the communication party during the handover in the BS and transmitting the packets to the MS after completion of the handover process.
However, in the above-mentioned method, the ACKs with respect to the packets transmitted from the MS are also buffered, and therefore, the arrivals of the ACKs to the MS delay regularly. In such a case, there is a high probability that the above-mentioned ACK response is received when the timer exceeds the RTO value (namely, probability that the time-out of the packet-retransmission timer occurs). In the result, the throughput is lowered by the above-mentioned slow-start algorithm after completion of the handover process.
In order to prevent such a lowering of throughput caused by arrival delays of ACKs, the communication party may delay transmissions of ACKs even in normal times so as to prolong the above-mentioned time to the time-out sufficiently for executing the handover processes, for example. However, in such a case, the MS is forced to be in an extra ACK-waiting mode regularly, which leads to the throughput lowering even in normal times.